For a mass analyzer operating under a scanning mode (such as a quadrupole) or under a pulse mode (such as time of flight, an electrostatic ion trap, etc.), when a flow of ions having a wide mass to charge ratio range is analyzed, ions outside a specific range of mass to charge ratios may be subjected to strength discrimination or cannot be used due to the inconsistency between the mass to charge ratio range of ions that can be analyzed instantaneously by the mass analyzer and that of the flow of ions, which may greatly affect the sensitivity and mass discrimination of mass spectrometers using these mass analyzers, such as a triple quadrupole, a tandem quadrupole-time of flight mass spectrometer or an electrostatic Orbitrap mass spectrometer. The traditional way to solve this problem includes:
A. Using an ion storage device to store the ions and discharging the ions synchronously according to the requirements of a mass analyzer of a subsequent stage.
B. Adding a mass-selective pseudo potential barrier or a fringe field structure at an end part of an ion guide, or modulating the ion ejection in conjunction with mass-selective resonances.
C. Using an additional ion guide or storage structure to temporarily store ions of a preceding stage in the time of flight analyzer, etc., and carrying out ion release and analysis according to its operating time sequence.
D. Using additional acceleration and deceleration lenses to ensure that the ions are sequentially synchronous with a time sequence of the following-stage mass analyzer at a controlled time.
However, the above methods has limitations:
As for A, a linear ion trap described in U.S. Pat. Nos. 7,208,728, 7,323,683 and a so-called Scanwave™ technology described in U.S. Pat. No. 9,184,039 are taken as an example. In such a mode, the ions are directly constrained by a DC potential produced by a plurality of axially arranged electrodes or by a radio-frequency pseudo potential. In addition, in this mode, axial transport control and mass-selective ejection of the ions are controlled by the same potential barrier formed axially, and the ion ejection and mass separation occur in the same direction. Since any ion storage device has a certain storage limit, the potential barrier has non-linear responses to mass selection when the ion flow exceeds the limit. Besides, the storage device itself may cause trailing, post-heating of the released ions due to the presence of a gas pressure or bound radio-frequency, and there are restrains on the extra high vacuum of a high-resolution mass analyzer, such that a certain transition distance generally exists between the analyzer and the ion storage device. Even though the released ions are synchronous with the time sequence of the following-stage mass analyzer, the mass discrimination occurs again due to different speeds of ions of different mass to charge ratios after the transition distance has been traveled.
B. Taken as an example is a secondary quadrupole DC potential well established in a length direction of an ion optical device through a multi-discrete electrode structure as described in U.S. Pat. Nos. 8,227,151, 8,487,248, etc., or a pseudo potential barrier featuring mass separation which is formed by using multiple spatial radio-frequency potential waveforms of different wavelengths through introducing an axial periodic electrode structure as described in U.S. Pat. Nos. 8,299,443, 9,177,776. In these methods, the mass separation potential barrier is axially positioned with respect to the ion transfer, and its fringe field structure itself may damage cooling and mass characteristics of the ions in a field axis. For quick ejection of the ions, an axial resonance excitation means that is introduced may enable greater energy distribution of the ions in an ejection direction, which may destroy resolution characteristics of high-resolution analyzers such as the quadrupole, time of flight and electrostatic ion trap analyzers, due to the deterioration of initial phase space distribution.
C. U.S. Pat. No. 7,582,864 is taken as a representative, in which an on-axis radio-frequency potential is achieved by using a two-phase amplitude-asymmetric radio frequency, and by combining the radio-frequency potential with a multipole field of electrodes induced by an end DC, ions are ejected in an order from large to small in terms of axial mass to charge ratio. However, such a guide or storage structure itself easily damages the perfection of the field of the analyzer due to the axial non-zero radio-frequency potential, thereby adding to the complexity of conditions required for subsequent ion focusing. Furthermore, asymmetric radio-frequency waveforms required by the guide or the storage structure may cause deterioration of the energy and spatial distribution of the ions upon release of the ions.
D. U.S. Pat. No. 8,754,367 is taken as a representative, in which a time-varying electric field is used firstly to separate ions of different mass to charge ratios, then its spatial position is used for constructing a non-linear electric field acceleration so as to allow the ions to finally enter an acceleration area of the time of flight at the same time. Although the ions may be well focused axially by this means, the axial non-linear electric field is inevitably accompanied by a huge non-linear divergent electric field radially according to the Laplace equation for electric field distribution. According to Liouville theorem, the temporal distribution of ions is compressed by this method, but sacrifices of radial space and energy focusing characteristics are inevitable, which is extraordinarily disadvantageous to high-resolution quadrupole, time of flight and electrostatic ion trap analyzers.